1. Field of the Invention
The present invention relates to a semiconductor-mounting composite heat-sink base having high heat dispersion characteristics and capable of being highly reliably bonded with a plastic package or flexible printed wiring board.
2. Description of the Prior Art
Recently, with improvements in the art of plastic packaging and/or flexible printed wiring, including higher density wiring, hermetic sealing, and faster response to signals, it has become possible to mount semiconductor devices of higher integration, faster performance, and larger capacity into units as LSI and IC packages. However, higher integration and faster operations involve increased heat generation from semiconductor elements, and this gives rise to an important problem that heat generated by the semiconductor elements be dispersed in order that the elements may normally operate.
Hitherto, as a solution to this problem, there has been proposed a plastic package or flexible printed wiring board of the type having a semiconductor mounting portion with a copper or copper alloy plate used as a heat sink. Typical examples of such arrangement are shown in section in FIGS. 10 and 11.
In FIGS. 10 and 11, numerals 19, 20 designate a copper or copper alloy-made base; 5 designates a multilayer plastic substrate formed of, for example, polyimide; 6 designates an Si semiconductor device mounted on the substrate 19, 20; and 21, 22, and 23 respectively designate a copper pin, a bonding wire, and a lid made of polyimide. The heat sink base 19 used in the package shown in FIG. 10 is of a flat plate configuration, and the heat sink base 20 used in the package shown in FIG. 11 is of a stepped configuration such that a semiconductor device-mounting portion is positioned higher so that the height of the semiconductor device 6 can be made even with the height of a terminal attachment portion of the multilayer plastic substrate 5, which provides an advantage over the FIG. 10 arrangement for purposes of semiconductor packaging.
With such packaging arrangement, the problem of heat dispersion can be solved. However, the fact that a semiconductor device has a thermal expansion coefficient of 4.2.times.10.sup.-6 (deg.sup.-1), whereas the thermal expansion coefficient of copper is 17.times.10.sup.-6 (deg.sup.-1), poses a problem that there is a substantial difference in thermal expansion between base 19, 20 and semiconductor device 6. As such, in either one of the two constructions shown there is a problem that bonding is adversely affected by thermal stress at an interface with respect to the semiconductor device mount, which will render the bonding unreliable.
In place of aforementioned copper and copper alloy, the use of a Cu--W or Cu--Mo composite alloy having lower thermal expansion characteristics and higher thermal conductivity has been considered. Typical constructions of plastic packages using a heat-sink base made of such a Cu--W or Cu--Mo composite alloy are schematically shown in section in FIGS. 12 and 13.
Use of such a construction has solved the problem of thermal expansion difference between Cu--W or Cu--Mo and a semiconductor device. However, a wire-breaking problem has come up to the fore with respect to fine copper wiring patterns. Therefore, in an attempt to prevent the breaking of fine copper wirings, it has been proposed to use a plastic substrate 5 having a thermal expansion coefficient of 13 to 17.times.10.sup.-6 (deg.sup.-1). In this case, however, there is still a problem such that cracking is likely to occur at a bonding interface between the substrate and the Cu--W or Cu--Mo composite heat-dispersion alloy 24, 25 due to a difference in thermal expansion therebetween, which is a problem from the standpoint of hermetic sealing.
As stated above, where a single material of copper or copper alloy, or of Cu--W or Cu--Mo composite ally is used as a heat-sink base for a plastic package or flexible printed wiring board, the problem is that the difference in thermal expansion involved will unfavorably affect the reliability of the semiconductor device or package as to the hermetic sealing thereof.
In order that this problem may be solved, it is necessary to equalize thermal expansion at a bonding interface. Thus, an idea has occurred that a composite material comprising copper or copper alloy and a Cu--W or Cu--Mo composite alloy bonded together be used as a heat-sink base for a plastic package or a flexible printed wiring board-mounting package.
Then, the advisability of using a heat-sink base comprising a copper plate and a Cu--W composite alloy joined by soldering, as shown in FIG. 14, was considered. To explain, by way of example, particulars studied, reference is had to FIG. 14 in which numeral 26 designates a Cu--W composite alloy of 20 mm.times.20 mm.times.1.0 mm having a copper content of 15 wt. %, 27 designates an oxygen-free copper plate of 30 mm.times.30 mm.times.1.2 mm, and 28 designates an Au--Sn eutectic solder of 20 mm.times.20 mm.times.0.05 mm having a melting point of 280.degree. C.
The reason why an Au--Sn eutectic solder having a melting point of 280.degree. C. is used is that since the temperature at which the heat-sink base is bonded with the plastic package or flexible printed wiring board is 260.degree. C., the heat-sink base is required to resist a temperature of 260.degree. C. or more. Further, it is intended that a residual stress, as well as distortion, due to the difference in thermal expansion between the copper plate (thermal expansion coefficient: 17.times.10.sup.-6 (deg.sup.-1)) and the Cu--W composite alloy (thermal expansion coefficient: 7.times.10.sup.-6 (deg.sup.-1) is minimized.
The Cu--W composite alloy plate 26 and oxygen-free copper plate 27, after Ni, Au plated, were placed in a reduction atmosphere at 300.degree. C. and were joined together by Au--Sn eutectic soldering. Thus, a semiconductor-mounting composite heat-sink base having a stepped configuration as shown by 29 was obtained. In this case, however, the heat-sink base had, on surfaces shown by 30 and 31, a warpage of 0.2 mm to 20 mm due to a difference in thermal expansion between the copper plate and the Cu--W composite alloy despite the fact that joining was effected by using a low melting point solder. Such warpage hindered the work of assembling the heat-sink base into unity with a plastic package or flexible printed wiring board, and did not permit sufficient bonding of surface 30 with semiconductor elements mounted thereon, with the result that the heat-sink base suffered from considerable deterioration in its heat dispersion characteristics.
Another problem was that where a Cu--W or Cu--Mo composite alloy plate 32 is joined with a copper or copper alloy plate 33 via brazing filler material 34 as shown in FIG. 15, the area of surfaces bonded is so large that a gas produced from the brazing filler material or residual air in the bond interface will cause a defect to a bond layer on a composite base 35 as shown in section. Thus, the heat-sink base cannot provide any good heat dispersion characteristics.